The present invention relates to an optical add/drop multiplexer used in a wavelength multiplexing technology, and more particularly to an optical add/drop multiplexer that transfers wavelength number information without any error.
With the higher capacity of a data communication represented by the Internet technology, a rapid increase in the information content and an increase in a transmission capacity are expected in an optical transmission system, and in order to meet the above demands, there is applied a wavelength multiplexing technology that bundles plural optical signals different in the wavelength in one optical fiber to conduct a communication. In recent years, there is being constituted a communication network using an optical add/drop multiplexer that not only conducts a high-capacity transmission between two points apart from each other by using the wavelength multiplexing technology, but also adds and drops an optical signal in each of one wavelengths between plural points.
A part of the communication network using the optical add/drop multiplexer will be described with reference to FIG. 1. FIG. 1 is a block diagram showing a communication system. FIG. 1 shows a structure in which sections of an optical add/drop multiplexer 101-1 and an optical add/drop multiplexer 101-2 are extracted from an optical network of a ring topology. The optical add/drop multiplexer 101-1 shown in the figure is only a device structure at an east side. In FIG. 1, solid lines indicate the flow of a main signal, and dotted lines indicate the flow of an OSC signal. Also, dashed lines indicate a supervisory channel electric signal.
First, the optical add/drop multiplexers 101-1 and 101-2 are made up of an optical amplifier section (west) 202-2, an optical amplifier section (east) 202-1, an optical add/drop multiplex section (west) 201-2, and an optical add/drop multiplex section (east) 202-1. The optical amplifier section 202 includes a receiver optical amplifier 203 that amplifies an input optical signal from an optical fiber transmission line 102, and transmits the amplified input optical signal to the optical add/drop multiplex section 201, and a transmitter optical amplifier 204 that amplifies the input optical signal from the optical add/drop multiplex section 201, and transmits the amplified input optical signal to the optical fiber transmission line 102. The optical add/drop multiplex section 201 includes an optical drop section having an optical coupler 206-1 and an optical demultiplexer 207, and an optical transmission/add select section having an optical demultiplexer 207, an optical multiplexer 208, optical switches 209, variable optical attenuators 210, optical couplers 206-2 that drop a part of an optical output after the variable optical attenuator 210, and photo detectors 214 that monitor a dropped optical signal of the optical coupler 206-2.
Hereinafter, a description will be given of the operation of the entire optical add/drop multiplexer 101 with reference to the flow of a main signal in a direction of from west to east in the optical add/drop multiplexer 101-2 shown in FIG. 1. The received optical signal from the optical add/drop multiplexer 101-1 is amplified by the receiver optical amplifier 203 of the optical amplifier section (west) 202-2 in the optical add/drop multiplexer 101-2, and is then transmitted to the optical add/drop multiplex section (west) 201-2.
The optical add/drop multiplex section (west) 201-2 branches the optical signal into two optical signals by the optical coupler 206-1, and one of those optical signals is further branched into optical signals in each of the wavelengths by the optical demultiplexer 207, and output from a dropped optical signal port. The other optical signal is transmitted to the optical add/drop multiplex section (east) 201-1 through an optical fiber 211 that connects between the optical add/drop multiplex sections 201.
The optical add/drop multiplex section (east) 201-1 is demultiplexed to optical signals in each of the wavelengths by the optical demultiplexer 207, and input to the optical switches 209. The optical switches 209 select a transmitted optical signal from west or an added optical signal. The variable optical attenuators 210 that are disposed downstream of the optical switches 209 are disposed in order to uniform the optical power levels of the respective wavelengths over the entire wavelengths, and the photodetectors 214 monitor the optical levels, and the optical levels of the respective channels are held constant under the control. The optical signals having the optical power levels uniformed by the variable optical attenuators 210 are again wavelength multiplexed by the optical multiplexer 208, and then transmitted to the optical amplifier section (east) 202-1.
In the optical amplifier section (east) 202-1, the optical signal is amplified by the transmitter optical amplifier 204, and thereafter again transmitted to the optical fiber transmission line 102.
Hereinafter, the optical supervisory channel (OSC) signal of the optical add/drop multiplexer will be described. The optical supervisory channel signal of the optical add/drop multiplexer functions to transfer the used wavelength number of the main signal between the optical add/drop multiplexers 101 so as to normally operate the optical amplifier 202. The function of transferring the wavelength number between the optical add/drop multiplexers is essential in the case where the optical amplifier implements the optical level constant control that holds the optical signal per one wavelength constant on the basis of the wavelength number information. Under the optical level constant control, because the optical output level that is a target of the optical amplifier is determined according to the wavelength number information, there occurs a difference between the optical output level that is actually required and a target optical output level that is calculated on the basis of the erroneous wavelength number information, which lead to an error in the main signal.
Referring to FIG. 1, the receiver optical amplifiers 203 operate under the optical level constant control. Wavelength number information acquisition and transfer sections 213 detect the wavelength number information within the optical add/drop multiplexer, and notify the detected information to OSC signal transmitter/receiver sections 212. Also, the wavelength number information acquisition and transfer sections 213 receive the wavelength number information that has been transferred from the optical add/drop multiplexer of another node from the OSC signal transmitter/receiver sections 212, and notify the receiver optical amplifiers 203 of the received information. The OSC signal transmitter/receiver sections 212 convert the wavelength number information of the node which has been transmitted from the wavelength number information acquisition and transfer sections 213 into an optical signal, and transmit the converted optical signal to a downstream optical add/drop multiplexer 101. Also, the OSC signal transmitter/receiver sections 212 receive the OSC signal that has been transmitted from an upstream optical add/drop multiplexer 101 through the transmission line 102, extract the wavelength number information from the OSC signal, and transmit the wavelength number information to the wavelength number information acquisition and transfer sections 213 as an electric signal. The OSC signals are separated from the main signal by a optical coupler 206-4 upstream of the receiver optical amplifier 203, and multiplexed with the main signal by the poststage optical coupler 206-3 downstream of the transmitter optical amplifier 204.
The transfer of the wavelength number information will be described with reference to FIGS. 2A and 2B. FIGS. 2A and 2B are block diagrams for explaining the transfer of the wavelength number information. FIG. 2A is a diagram that extracts a function of transferring the wavelength number from the east direction of the optical add/drop multiplexer 101-1 to the west direction of the optical add/drop multiplexer 101-2 from the communication system that is described with reference to FIG. 1. FIG. 2B is a diagram for explaining the flow of a main signal.
Referring to FIG. 2A, alphabets A to E represent operating order in the wavelength number transfer control. Referring to FIG. 2B, in the optical add/drop multiplexer 101-1, the optical signals of wavelengths λ1 and λ2 are multiplexed by the optical add/drop multiplex section (east) 201-1, and an optical signal of a wavelength λ3 is transmitted through the optical add/drop multiplexer 101-1. In this case, the optical level can be first observed at a position A by only the optical level monitor 214 of λ1, λ2 and λ3, and the optical level of other wavelengths cannot be observed. Accordingly, the wavelength information acquisition and transfer section 213 recognizes that the wavelength number is observed by three wavelengths at the position A from the optical level monitors (photo detectors) 214. The wavelength number information of the three wavelengths is transferred to the OSC signal transmitter/receiver section 212 from the wavelength information acquisition and transfer section 213 at a position B. The OSC signal transmitter/receiver section 212 converts the wavelength number information of the three wavelengths which has been electrically received at the position B into an optical signal, and transmits the optical signal at a position C. The optical signal that has been transmitted at the position C is multiplexed with the main signal by the optical coupler 206-3 of the optical amplifier (east) 202-1, and then transmitted to the optical add/drop multiplexer 101-2. The optical add/drop multiplexer 101-2 first demultiplexes the OSC optical signal and the main signal by the optical coupler 206-4, and the OSC signal transmitter/receiver section 212 receives the OSC optical signal. The OSC signal transmitter/receiver section 212 converts the received OSC optical signal into an electric signal from the optical signal at a position D, and receives the wavelength number information of the three wavelengths which has been acquired by the optical add/drop multiplexer 101-1. The received wavelength number information is transmitted to the wavelength number information acquisition and transfer section 213 at a position E. The wavelength number information acquisition and transfer section 213 transmits the received wavelength number information to the receiver optical amplifier 203 of the node at a position F. Upon receiving the wavelength number information, the receiver optical amplifier 203 determines a target value of the output optical level on the basis of the wavelength number information of the three wavelengths, and conducts the optical amplification of the main signal so as to meet the target output level.
Japanese Patent Laid-Open No. 2005-286721 discloses a technique in which in order to prevent the wavelength of an add optical signal from being erroneously set, a part of a optical signal that is multiplexed in wavelength by a wavelength multiplexer is reflected by a reflector, and detected by a backward detector of the wavelength multiplexer that multiplexes the add optical signal. United States Patent Application Publication 2005/0226621 A1 is a counterpart application of JP Patent Laid-Open No. 2005-286721.
In the above background art, the wavelength number is counted up in a wavelength demultiplexed state. For that reason, there may occur a difference between the wavelength division multiplexed wavelength number and the wavelength number that is normally multiplexed in the wavelength. More specifically, when the optical multiplexer is in failure or the wavelength of the added optical signal is in error, there occurs an error in the wavelength number.
The technique disclosed in Japanese Patent Laid-Open No. 2005-286721 is capable of detecting the wavelength error of the added optical signal, but a large number of optical parts are required. Also, there is no disclosure of the failure of the optical multiplexer.